CVS takes cells from the placenta. Amnio takes cells from the amniotic fluid that have been shed from the baby.


CVS
Baby and placenta are both produced from the original egg + Sperm. The sperm enters the egg and then these split into 2 cells, and then 4 cells....

Presumably, all these cells are identical to the original cell, but sometimes there are slight changes.

Early in this splitting, the cells that produce the baby are separated from those that produce the placenta. The baby cells keep dividing making the baby. The cells that go to make the placenta also keep dividing, but are no longer part of those that are making baby.

It is possible that something happens to one of the cells that are dividing to make the placenta, that do not affect the cells in the baby. This cell will keep dividing and will form part of the placenta.

When a CVS is performed there is the chance that this 'change' that has occurred (e.g. an extra chromosome or minus a chromosome) will be detected in the placenta even though the change is not present in the baby.

If this happens, the lab will see some cells that look like the baby (the cells without the change) and some cells that will have the change.

The lab knows that some changes only occur in placenta cells (if it happened in a baby cell, the baby would not keep growing) and therefore they know that they don't have to worry about these changes.

Some changes detected through CVS in only some cells are harder to work out if they will affect the baby because they would still produce a viable baby (e.g. 3 copies of chromosome 21).

If some 'normal' cells and some cells with changes were detected through CVS, they might suggest to the Mum that she have a follow up amnio to confirm.

This would be very rare, but it is a possibility.

For a CVS to have completely 'wrong' results (ie the results from the cells taken from the placenta are not the same as the cells that make up the baby), a change in a cell must have occurred very early in a cell that makes the placenta. and this cell, must have made all the subsequent cells in the placenta and have contributed no cells to the baby.

This situation is almost so so so so rare to occur that i don't think it has ever been detected yet.

Amnio

The cells analysed using amnio are closer (than CVS) to the composition of the cells in the baby, because the cells they look at come from the fluid surrounding the baby. So these cells have come from the baby and have had less time to have changes occur (that aren't already present in the baby).

Summary

So, it is possible for CVS to produce uncertain results but this is rare, and the lab can usually tell whether the changes detected will affect the bub. Overall, both CVS and amnio are in all but the rarest times quite definitive in the results.